Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks

Stefan Dollhopf; Ingo Weidlich

doi:10.1007/978-3-032-09844-3_11

Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks

^*Corresponding author for this work

Abstract

This study investigates the thermal conductivity of backfill materials for buried bonded pipe systems, with a focus on comparing conventional natural aggregates and recycled alternatives. Three natural and three recycled materials were tested under compacted conditions using a thermal needle probe in accordance with ASTM D5334-22a. Thermal conductivity was continuously recorded and correlated with volumetric water content. A classification methodology based on EN 13,941–1 was developed to assign thermal conductivity values to defined moisture levels (dry, medium, wet). The results show that the investigated natural materials exhibit higher thermal conductivity across all moisture states, while the investigated recycled materials demonstrate significantly lower values. Through this two-fold approach—measurement and classification—representative parameters were derived for future application in heat loss calculations. In addition, the findings could improve the understanding of the thermal interaction between soil and pipe systems, supporting the potential integration of alternative materials into sustainable infrastructure design.

Original language	English
Title of host publication	Proceedings of the 19th International Symposium on District Heating and Cooling
Subtitle of host publication	The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC
Editors	Dirk Vanhoudt
Pages	109-118
Number of pages	10
Volume	1700
DOIs	https://doi.org/10.1007/978-3-032-09844-3_11
Publication status	Published - 24 Jan 2026
Event	19th International Symposium of District Heating and Cooling, IEA DHC 2025 - Genk, Belgium Duration: 7 Sept 2025 → 10 Sept 2025

Publication series

Name	Lecture Notes in Networks and Systems
Volume	1700
ISSN (Print)	2367-3370
ISSN (Electronic)	2367-3389

Conference

Conference	19th International Symposium of District Heating and Cooling, IEA DHC 2025
Country/Territory	Belgium
City	Genk
Period	7/09/25 → 10/09/25

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

SDG 07 Affordable and Clean Energy

Keywords

Recycled materials
Sustainable construction methods
Thermal soil properties

Access to Document

10.1007/978-3-032-09844-3_11Licence: CC BY

Cite this

Dollhopf, S., & Weidlich, I. (2026). Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks. In D. Vanhoudt (Ed.), Proceedings of the 19th International Symposium on District Heating and Cooling : The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC (Vol. 1700, pp. 109-118). (Lecture Notes in Networks and Systems; Vol. 1700 ). https://doi.org/10.1007/978-3-032-09844-3_11

Dollhopf, Stefan ; Weidlich, Ingo. / Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks. Proceedings of the 19th International Symposium on District Heating and Cooling : The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC. editor / Dirk Vanhoudt. Vol. 1700 2026. pp. 109-118 (Lecture Notes in Networks and Systems).

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title = "Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks",

abstract = "This study investigates the thermal conductivity of backfill materials for buried bonded pipe systems, with a focus on comparing conventional natural aggregates and recycled alternatives. Three natural and three recycled materials were tested under compacted conditions using a thermal needle probe in accordance with ASTM D5334-22a. Thermal conductivity was continuously recorded and correlated with volumetric water content. A classification methodology based on EN 13,941–1 was developed to assign thermal conductivity values to defined moisture levels (dry, medium, wet). The results show that the investigated natural materials exhibit higher thermal conductivity across all moisture states, while the investigated recycled materials demonstrate significantly lower values. Through this two-fold approach—measurement and classification—representative parameters were derived for future application in heat loss calculations. In addition, the findings could improve the understanding of the thermal interaction between soil and pipe systems, supporting the potential integration of alternative materials into sustainable infrastructure design.",

keywords = "Recycled materials, Sustainable construction methods, Thermal soil properties",

author = "Stefan Dollhopf and Ingo Weidlich",

note = "Publisher Copyright: {\textcopyright} The Author(s) 2026.; 19th International Symposium of District Heating and Cooling, IEA DHC 2025 ; Conference date: 07-09-2025 Through 10-09-2025",

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Dollhopf, S & Weidlich, I 2026, Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks. in D Vanhoudt (ed.), Proceedings of the 19th International Symposium on District Heating and Cooling : The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC. vol. 1700, Lecture Notes in Networks and Systems, vol. 1700 , pp. 109-118, 19th International Symposium of District Heating and Cooling, IEA DHC 2025, Genk, Belgium, 7/09/25. https://doi.org/10.1007/978-3-032-09844-3_11

Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks. / Dollhopf, Stefan ; Weidlich, Ingo.
Proceedings of the 19th International Symposium on District Heating and Cooling : The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC. ed. / Dirk Vanhoudt. Vol. 1700 2026. p. 109-118 (Lecture Notes in Networks and Systems; Vol. 1700 ).

Research output: Chapter in Book (Inbook) /Conference Proceeding › Conference Paper › Research Article/Contribution › peer-review

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AU - Dollhopf, Stefan

AU - Weidlich, Ingo

N1 - Publisher Copyright: © The Author(s) 2026.

PY - 2026/1/24

Y1 - 2026/1/24

N2 - This study investigates the thermal conductivity of backfill materials for buried bonded pipe systems, with a focus on comparing conventional natural aggregates and recycled alternatives. Three natural and three recycled materials were tested under compacted conditions using a thermal needle probe in accordance with ASTM D5334-22a. Thermal conductivity was continuously recorded and correlated with volumetric water content. A classification methodology based on EN 13,941–1 was developed to assign thermal conductivity values to defined moisture levels (dry, medium, wet). The results show that the investigated natural materials exhibit higher thermal conductivity across all moisture states, while the investigated recycled materials demonstrate significantly lower values. Through this two-fold approach—measurement and classification—representative parameters were derived for future application in heat loss calculations. In addition, the findings could improve the understanding of the thermal interaction between soil and pipe systems, supporting the potential integration of alternative materials into sustainable infrastructure design.

AB - This study investigates the thermal conductivity of backfill materials for buried bonded pipe systems, with a focus on comparing conventional natural aggregates and recycled alternatives. Three natural and three recycled materials were tested under compacted conditions using a thermal needle probe in accordance with ASTM D5334-22a. Thermal conductivity was continuously recorded and correlated with volumetric water content. A classification methodology based on EN 13,941–1 was developed to assign thermal conductivity values to defined moisture levels (dry, medium, wet). The results show that the investigated natural materials exhibit higher thermal conductivity across all moisture states, while the investigated recycled materials demonstrate significantly lower values. Through this two-fold approach—measurement and classification—representative parameters were derived for future application in heat loss calculations. In addition, the findings could improve the understanding of the thermal interaction between soil and pipe systems, supporting the potential integration of alternative materials into sustainable infrastructure design.

KW - Recycled materials

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BT - Proceedings of the 19th International Symposium on District Heating and Cooling

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Dollhopf S , Weidlich I. Experimental Investigation on the Thermal Conductivity of Alternative Backfill Materials for District Heating Networks. In Vanhoudt D, editor, Proceedings of the 19th International Symposium on District Heating and Cooling : The International Research Conference on Heating and Cooling Networks - Under the Supervision of IEA DHC. Vol. 1700. 2026. p. 109-118. (Lecture Notes in Networks and Systems). doi: 10.1007/978-3-032-09844-3_11